For this you need to press the F3 key and you will see a checkbox ticked in the top left saying: "Disable Console". Press F8 to get a mouse cursor and uncheck the box to activate the console. Once the console is enabled, press "Enter" then type in the command. Whenever you restart the game, you need to press one of the overlay keys first (e.g. F3) before pressing the console key.

Effect	Code
All blueprints unlocked	ALLBLUEPRINTS
Can go faster or slower	SPEED
Faster building	FASTBUILD
Free useful items	MADLOOT
Invincibility	NODAMAGE
No Radiation	RADIATION
No crafting/building costs	NOCOST
Set the speed of the day speed and the night speed	DAYNIGHTSPEED
Set to day	DAY
Set to night	NIGHT
Spawn Cyclops	SUB CYCLOPS
Spawn Seamoth	SPAWN SEAMOTH 1
Spawn item	ITEM (NAME OF ITEM) (AMOUNT)
Teleport to biomes	BIOME (Name)
Teleport to safety	WARPME
Unlimited Oxygen	OXYGEN